Bug 1329110. Part 6 - remove StateObject::HandleWaitingForData().
Since DecodingState is the only one that overrides the function, we will let it
just override HandleWaitingFor{Audio,Video}. We also reduce the code complexity
because it is hard to trace the code when one virtual function calls another
virtual function.
MozReview-Commit-ID: AdLXpDgvOyx